A major problem of sensors, which measure in a certain medium, especially liquid, is their fouling. Such fouling is brought about by deposits, biofilms, etc. In the case of almost all sensor types (electrochemical, biochemical, optical, . . . ), such contaminations can lead to wrong measured values and even complete failure of the sensor, wherein, above all, the fouling of membranes, electrodes, active surfaces, optical windows, . . . must be classed as critical.
By way of example, this will be explained based on a turbidity sensor. Fundamentally, the problem arises, however, in the case of all sensors, which measure in liquid and require a certain cleanliness of certain sensor components. Especially to be mentioned here are also similar process variables such as solids content or sludge or slurry level. Measuring devices, which are suitable for determining the corresponding process variables, are manufactured and sold in large variety by the group of firms, Endress+Hauser.
Usually, the sensors are arranged in a sensor housing, and the determining of the process variable happens frequently optically. In such case, electromagnetic waves of a certain wavelength are transmitted from at least one transmitting unit, scattered by the medium to be measured and received by a receiving unit. The measured parameter is deduced as a function of the scattering.
The electromagnetic waves pass through the housing through a material transmissive for the electromagnetic waves, frequently a corresponding window. In the course of time, fouling occurs on the window and the electromagnetic waves can no longer pass through (unimpeded). The measurements become corrupted and the sensor can even fail completely.
In order to maintain the functional ability of the sensor, the window must be cleaned.
The following methods are known for cleaning: Manual, per wiper, with air, by rinsing or washing, and by means of oscillatory or ultrasonic transducers situated in the housing.
In the case of manual cleaning, the sensor must be removed from the process. In case the application involves a pipeline, this requires either that the process must be paused or an expensive retractable assembly used. For hygienic applications, wiper-, air- and rinse or wash cleaning cannot be used, due to dead spaces.
German Patent, DE 43 17 068 C2 shows an ultrasonic transmitter mounted directly on the window. In this way, not only the window, but also the housing, is caused to oscillate. As a rule, the installation space for an ultrasonic transmitter in the sensor is very limited. Because of this, low powered transmitters must be used. Additionally, the energy transmission of the ultrasound into the liquid is not optimal due to the window pane thickness. This leads, in combination with the limited electrical power available in the sensor, to a small cleaning action.
German Patent, DE 90 15 235.2 discloses a turbidity sensor with windows, wherein the windows are kept continuously in oscillation by an oscillatory transducer. As a result of this, highly accurate measurements are not possible.
U.S. Pat. No. 5,889,209 shows an immersible ultrasonic transmitter for a sensor for dissolved oxygen in aqueous media located in relatively open containments such as in a river, in the ocean or in a pond. Ultrasonic transmitter and sensor are located both completely under water and the ultrasonic transmitter transmits, at a small spatial separation of 4 mm to 10 mm from the sensor and a time interval of 5 min to 120 min, ultrasonic waves with a duration of 5 s to 90 s in the direction of the membrane of the sensor. Because of the complete immersion and the water contact, hygienic applications seem out of the question in such case.
German Patent, DE 198 11 876 B4 discloses an oscillation producer, which is coupled mechanically with the window, wherein the window is suspended elastically in the housing. For producing the oscillations, most often, a piezo electrical transducer is used. Large voltages and electrical currents are necessary for its operation. It is clear, thus, that in the immediate vicinity of the measuring system there will be electromagnetic fields, which can lead to EMC problems, especially as regards the measuring electronics. As a rule, the installation space for an ultrasonic transmitter in the sensor is very limited. Because of this, low power transmitters must be used. Additionally, the energy transfer of the ultrasound into the liquid is not optimal due to the window pane thickness. This leads, in combination with the limited electrical power available in the sensor, to a small cleaning action.